1. Field of the Invention
This invention relates in general to sensors, more particularly to acoustic sensors, and most particularly to acoustic sensors for underwater applications.
2. Description of the Related Art
In the past, acoustic transducers and hydrophones have been used to convert underwater acoustical energy into electrical signals. These are typically ceramic-based products that work on the principle of either piezoelectricity or magnetostriction. These devices are typically capable of both transmitting and detecting acoustic pressure in the water. However, the receiver aperture and bandwidth of these devices are limited.
Laser-based techniques for detection of sound for underwater applications have also been developed. These include optical fiber schemes and alternatively, laser interrogation of a water column. Optical fiber arrangements detect underwater sound by monitoring changes in the optical fiber properties (optical or physical) due to an incident acoustic pressure wave. Some arrangements normally require a polished optical fiber to deliver light in close proximity to a membrane similar to an optical fiber microphone. These devices also require obtaining properties such as the Fresnel reflection at the laser output (fiber end) and an acoustic collector unit. Other devices coil the optical fiber around a mandrel that stretches or compresses the fiber when subjected to an acoustic pressure field. [reference: C. Davis, E. Carome, M. Weik, S. Ezekiel, and R. Einzig, “Fiberoptic Sensor Technology Handbook’, Optical Technologies Inc., ch. 5, 1986.] The strain on the optical fiber affects the phase of the light propagating within the fiber.
Alternatively, laser light, typically near the 500 nanometer wavelength region, is directed into the water and reflected from particles within the water vibrating in response to an acoustic pressure wave. The reflected light is processed to provide detected sound pressure levels and the bearing of the sound source in the water environment.
More recently, laser-based techniques for specific underwater applications for acoustic detection have been developed. U.S. Pat. No. 6,188,644 describes using a laser Doppler vibrometer to measure vibrations from an air-water boundary located in the water volume in order to detect underwater sound. The air-water boundary creates a free surface that can extend within the water volume, therefore, not being constrained in shape or size. However, such an air-water boundary may only be achieved on the surface of a supercavitating object. Also, U.S. Pat. No. 6,349,791 describes an acoustic sensor assembly used in a submarine bow that employs a laser scanner deployed behind an acoustic panel, placed between the inner and outer hulls of the submarine. While such a configuration allows a user to obtain acoustic data from the outer hull of the submarine, such data may be inaccurate due to the normal placement of a transducer between the inner and outer hull surfaces of a submarine, which can create interference due to the acoustics reflecting between the outer hull and the transducer. Also, installation of a separate panel between the inner and outer hulls may be cumbersome and costly.
Therefore, it is desired to provide a laser-based acoustic sensor/transducer for a variety of underwater applications that is cost effective, easily installed, and does not interfere with other acoustic devices used by underwater objects.